Regulation of channels and transporters by ubiquitination. (a) ENaC ubiquitination by NEDD4-2. NEDD4-2 binds to PY motifs on the epithelial Na+ channel ENaC and catalyzes its ubiquitination. This induces ENaC endocytosis and lysosomal targeting, resulting in fewer channels at the cell surface. To increase Na+ transport, NEDD4-2 is phosphorylated by kinases, including PKA, SGK, and IKKβ, in turn activated by various signaling pathways. Phosphorylation of NEDD4-2 induces binding of 14-3-3 dimers (not shown), which prevents NEDD4-2 from binding to ENaC. As a result, endocytosis of ENaC is inhibited, and increased ENaC presence at the surface enhances epithelial Na+ absorption. (b) Rsp5 ubiquitinates permeases and transporters. In yeast, arrestin-related endocytic adaptors (ARTs) and the E3 ubiquitin ligase Rsp5 are recruited to the plasma membrane in response to environmental stimuli that trigger the endocytosis of proteins such as permeases and transporters (for example, the arginine transporter Can1). Through their PY motifs, ARTs bind to the WW domain of Rsp5 and mediate ubiquitination of cargo. ARTs are also ubiquitinated by Rsp5, an event required for endocytosis.